1. Field of Invention
The present invention relates to a metering device that measures total combined mass flow of multi phase fluids consisting of mixtures of gases and liquids in any proportion and even when slippage occurs between the gas and liquid or between two liquids. Slippage occurs when gas travels at a different velocity than the liquid in the line or one liquid travels at a different velocity than another liquid. The liquid can be homogeneous or can be a mixture of two or more miscible or immiscible liquids such as oil and water.
2. Background Information
In order to provide background information so that the invention can be completely appreciated in its proper context reference may be made to a number of prior art patents as follows: U.S. Pat. No. 5,301,557 assigned to Micromotion Inc., U.S. Pat. No. 5,287,754 assigned to Khrone Inc., U.S. Pat. No. 4,934,195 assigned to Foxboro Company.
All of the above mass flow meters utilize the Coriolis effect to measure single phase fluid which may be all liquid or all dense gas but have not been successfully used in measuring multi phase fluids such as a mixture of gas and liquid in the entire gas-liquid mixture range. The Coriolis force is produced when the radius of a mass traveling in angular motion is changed. The change in radius causes a corresponding change in angular acceleration which produces a net force tangent to the direction of rotation of the mass and opposite to the direction of rotation.
Present Coriolis mass flow meters rely on producing the Coriolis effect by vibrating the flow tube elements that is either full of liquid or dense gas at their natural frequency. The twist of the tubes is then translated to mass flow by some instrumentation means. However, when these vibrating tube flow meters are used in two phase (gas/liquid mixtures) measurements, vibration of the mixture of liquid and gas in the tubes causes continuously variable dampening forces which oppose harmonic motion. The oscillatory motion of the tubes causes the liquid part to be tossed back and forth or up and down in the gas space, such action causing a portion of the Coriolis force to be absorbed internally in the fluid instead of being transmitted to the tubes where the effect is measured. Because the liquid only occupies partial volume in the tube, oscillation causes the liquid to be displaced at a different rate than the gas because of different densities. This phenomenon causes gross inaccuracies in measurement which cannot be quantified or predicted. The dampening effect of the gas/liquid mixture cannot be quantified or predicted because the instantaneous gas/liquid ratio in the tubes is continuously varying with the flow. Dampening of the tube vibration caused by multi phase fluids also overloads the tube driver because harmonic motion cannot be established or maintained. This makes the instruments either inoperative or unbalanced and results in gross inaccuracies in measured flows or in no flow indications at all. All of these instruments measure the twist of the tubes and when a multi phase fluid is present, the motion of the tube in one direction will produce a different Coriolis force than the motion in the opposite direction which prevents harmonic motion. When the tubes fail to attain harmonic frequency, more excitation force is required which would overload the tube driver.
Other prior arts are U.S. Pat. No. 2,813,423 and U.S. Pat. No. 3,087,325 both titled Gyroscopic Mass Flowmeter issued Nov. 19, 1957 and Apr. 30, 1963 respectively. These devices are both made of conduit formed into loops and rotated or oscillated. When a multiphase fluid is passed through these loops, the centrifugal force will separate the liquid from the gas. The liquid will then settle at portions of largest rotational diameter and will create a liquid seal that will prevent flow similar to a U manometer. Since the liquid is subjected to centrifugal force, the pressure required to displace the seal will be much higher than when the liquid is not rotated. Therefore, this device can only be used for a single phase fluid such as pure liquid or pure gas.
Another prior art is U.S. Pat. No. 2,877,649 titled Coriolis Mass Flowmeter. This prior art also cannot measure multiphase fluids accurately because of the fluid path. The fluid flows from the principal impeller to the sensing impeller in a direction of decreasing radius of rotation. Such flow when the fluid consists of gas and liquid will be separated into gas and liquid because of centrifugal force, the liquid forming a liquid seal on the outer radius of rotation and the gas forming on the inside radius of rotation resulting in a liquid seal or U trap which would require a build up of pressure to dislodge the liquid seal. Also, because of the clearances between the principal and sensing impellers, some of the fluid can bypass the sensing impeller. The turbulence created by flowing from the principal to the sensing impeller also results in Coriolis force being absorbed by the fluid and render the measurement inaccurate.
One other prior art is U.S. Pat. No. 5, 359,900 titled Apparatus for Measuring the Mass Throughput of a Flow of Pourable Material According to the Coriolis Principle. This prior art has a torque measuring device that does not eliminate the force due to friction of the bearings and friction due to turbulence created by the rotating member. These forces would add to the Coriolis force and render the results inaccurate. This prior art refers to pneumatic conveying systems which do not require high pressures and therefore seals are not critical. When handling multiphase fluids such as gas, oil and water from oil wells, high pressures can result in high friction forces on mechanical seals and bearings. Also, the turbulence and fluid friction created by contact of the non-measuring surface of the rotating member with the fluid add to the torque being measured. Since these friction forces cannot be quantified, the measurement will be inaccurate. Also, the vane arrangement of the prior art will introduce turbulence into the fluid since the fluid is not directed to flow into a uniform cross section as embodied in the constant cross section of the conduits of the present invention.
Whatever the merits and features of the above cited references are, none of them achieves or fulfills the purposes of the present invention which is to measure accurately gas-liquid mixtures ranging from 0% liquid (all gas) to 100% liquid (no gas) and all the fractions of liquid and gas in between these ranges. When these prior art devices measure liquid mass flow, the presence of gas especially in the higher fractions in the flow stream makes these instruments highly inaccurate. It is the principal object of the present invention to be able to meter accurately any combination of gas and liquid from 0% liquid (all gas) to 100% liquid (no gas) within its designed mass flow rates and without regard to slippage between the gas and the liquid or between liquids if the liquid is a mixture of different types such as oil and water.
The present invention only measures the Coriolis force and is not affected by friction on mechanical seals and bearings and friction created by contact of the non-measuring outside surface of the rotating member and the fluid.